Chromium (chemical symbol Cr, atomic number 24) is a hard, shiny, steel-gray metal that takes a high polish and does not tarnish. It is therefore used in alloys, such as stainless steel, and in chrome plating. The human body needs trace amounts of trivalent chromium (chromium(III)) for sugarmetabolism, but hexavalent chromium (chromium(VI)) is very toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of rubies derives from the presence of chromium. Some compounds, particularly potassium and sodium dichromate, are oxidizing agents useful for the oxidation of organic compounds and (with sulfuric acid) for cleaning laboratory glassware. In addition, chromium(VI) oxide is used in manufacturing high-performance audiotapes.

Discovery and etymology

In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it "Siberian red lead." Though misidentified as a lead compound with selenium and iron components, the material was in fact lead chromate, with the chemical formula PbCrO4. It is now known as the mineral crocoite.

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red "lead" mineral that had very useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. In addition, a bright yellow made from crocoite became a fashionable color.

In 1797, Nicolas-Louis Vauquelin received samples of crocoite ore. By mixing crocoite with hydrochloric acid, he was able to produce chromium oxide, with the chemical formula CrO3. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven. He was also able to detect traces of chromium in precious gemstones such as ruby and emerald.

During the 1800s, chromium was primarily used as a component of paints and in tanning salts. Now its primary use is for metal alloys, accounting for 85 percent of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

Chromium was named after the Greek word "chroma" meaning color, because of the many colorful compounds made from it.

Occurrence and isolation

Chromium is mined as chromite (FeCr2O4) ore. Roughly half this ore in the world is produced in South Africa. In addition, Kazakhstan, India, and Turkey are substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they have been discovered. The Udachnaya Mine in Russia produces samples of the native metal. This mine is a kimberlite pipe rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.

To isolate the metal commercially, chromite ore is oxidized by reacting it with molten alkali (sodium hydroxide, NaOH). This produces sodium chromate (Na2CrO4), which is reduced with carbon to chromium(III) oxide (Cr2O3). The metal is obtained by heating the oxide in the presence of aluminum or silicon.

About 15 million tons of marketable chromite ore were produced in 2000 and converted into roughly four million tons of ferrochrome (consisting of 70 percent chromium alloyed with iron), with an approximate market value of 2.5 billion U.S. dollars.

Notable characteristics

In the presence of oxygen, chromium rapidly produces a thin oxide layer that protects the metal from further reaction with oxygen.

As a transition element, chromium combines with oxygen and other elements in several different ratios. Thus it forms compounds in which it has a variety of oxidation states. Its common oxidation states are +2, +3, and +6, with +3 being the most stable. In addition, the +1, +4, and +5 states have been observed in rare cases. Chromium compounds of oxidation state +6 are powerful oxidants.

Isotopes

The isotopes of chromium range in atomic weight from 43 atomic mass units (amu) (43Cr) to 67 amu (67Cr). Naturally occurring chromium is composed of three stable isotopes: 52Cr, 53Cr, and 54Cr. Of these, 52Cr is the most abundant (83.789 percent natural abundance). In addition, 19 radioisotopes have been characterized, with the most stable being 50Cr with a half-life greater than 1.8x1017 years. The isotope 51Cr has a half-life of 27.7 days, and all the other radioactive isotopes have half-lives under 24 hours; the majority of these have half-lives less than one minute. This element also has two meta states.

Chromium isotopic contents in the earth are typically combined with manganese (Mn) isotopic contents and have found application in isotope geology. The isotope 53Cr is produced by the radioactive decay of 53Mn. Isotope ratios for Mn/Cr reinforce other types of evidence for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites provides supporting evidence for the creation of new atomic nuclei immediately before coalescence of the solar system.

Compounds

Chromium(III) oxide

Chromium(III) oxide (Cr2O3) also known as chromium sesquioxide or chromia, is one of four oxides of chromium. It is manufactured from the mineral chromite, noted above. Green in color, it is commonly called chrome green when used as a pigment in enamel painting and glass staining. It can dissolve in acids to give chromium(III) salts, and in molten alkali to give chromites.

Potassium dichromate

Potassium dichromate (K2Cr2O7) is a powerful oxidizing agent and is the preferred compound for cleaning laboratory glassware of any possible organics. It is used as a saturated solution in concentrated sulphuric acid for washing the apparatus. (Sometimes, however, sodium dichromate is used for this purpose, based on its higher solubility.) In addition, it can drive the oxidation of organic compounds, as in converting a primary alcohol to an aldehyde and then to a carboxylic acid.

Potassium dichromate is one of the most common culprits in causing chromium dermatitis. Chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and forearms, which is chronic and difficult to treat. As with other Cr(VI) products, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection.

Chromic acid

Chromic acid has the hypothetical structure H2CrO4. Neither chromic nor dichromic acid is found in nature, but their anions are found in a variety of compounds. Chromium trioxide, CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid."

Lead(II) chromate

Lead(II) chromate (PbCrO4) has a vivid yellow color and is practically insoluble in water. For this reason, it was once commonly used as a paintpigment under the name "chrome yellow."

Chromium and the quintuple bond

Chromium is notable for its ability to form quintuple covalent bonds. Writing in the journal Science, Tailuan Nguyen, a graduate student working with Philip Power of the University of California, Davis, describes the synthesis of a compound of chromium(I) and a hydrocarbonradical.[1] This compound was shown (by X-ray diffraction) to contain a quintuple bond joining two chromium atoms.

The formula for the compound may be written as

Ar − Cr − Cr − Ar

where Ar represents a specific aromatic group.

Chromium currently remains the only element for which quintuple bonds have been observed.

Applications

Chromium and its compounds have a variety of applications, some of which are noted below.

In metallurgy, chromium imparts corrosion resistance and a shiny finish. It is therefore a constituent in alloys, such as in stainless steel used for cutlery. It is also used in chrome plating.

In anodized aluminum, chromium imparts a ruby color to the surface of aluminum.

Chromium(III) oxide is a metal polish known as green rouge. In addition, it is used as a pigment under the name chrome green, as noted above.

Potassium dichromate is an oxidizing agent used for oxidizing organic compounds such as alcohols and aldehydes, and for cleaning laboratory glassware (mentioned earlier). It is also used as a mordant (fixing agent) for dyeing fabric, and in photography and photographic screen printing.

Chromium(VI) oxide (CrO3) is used to manufacture magnetic tapes (such as audiotapes), which give better performance than iron oxide tapes.

Biological role

Trivalent chromium (Cr(III) or Cr3+) is required in trace amounts for sugarmetabolism in humans, and its deficiency can cause chromium deficiency. By contrast, hexavalent chromium (Cr(VI)) is very toxic.

Precautions

Chromium metal and chromium(III) compounds are not usually considered health hazards, but hexavalent chromium (chromium VI) compounds can be toxic if orally ingested or inhaled. Most chromium (VI) compounds are irritating to the eyes, skin, and mucous membranes. Chronic exposure to chromium (VI) compounds can cause permanent eye injury unless properly treated. In addition, chromium(VI) is an established human carcinogen. The lethal dose of poisonous chromium (VI) compounds is about one-half teaspoon of material. According to recommendations by the World Health Organization, the maximum allowable concentration of chromium (VI) in drinking water is 0.05 milligrams per liter.

As chromium compounds have been used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites that now need environmental cleanup and remediation. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications.

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